Two profound changes will soon modify medical ultrasound use: on-screen safety indexes (regulatory), and 2) the emergence of clinically-useful microbubbles for contrast enhancement. The safety indexes relate clinical ultrasound (US) fields to the potential for bioeffects. The Thermal Index concerns US-generated temperature increments in tissues. The Mechanical Index (MI) is defined in terms of peak negative acoustic pressure and concerns non-thermal bioeffects. Both indexes are based upon knowledge of the mechanisms of action of US fields on tissues/cells. The bioeffect and physical bases for the MI are limited; e.g., the MI does not acknowledge the importance of US pulse length for generating inertial cavitation (IC). Doppler US device output levels generally exceed those requisite for IC from micron-sized nuclei. The general aim of the project is to provide mechanistic insights on dynamic US-induced bubble interactions with in vitro and in vivo biological systems in relation to the MI. The overall hypothesis guiding the proposed project is that non-thermal US-induced cellular effects are due primarily to the action of bubbles. All projects involve the use of a stabilized microbubble echo contrast agent as a control on the presence of bubbles during insonation. SupportIng preliminary data or demonstrable expertise in the area of investigation exists for all projects. Ten hypotheses are proposed for testing, and deal with acoustic thresholds for: (1) pulsed US exposure conditions and hemolysis in vitro, (2) 'jet'-induced erosion of artificial and natural membranes, (3) increases in in vitro mammalian cell mutation and sister chromatid exchanges, (4) confirmation of US-induced potentiation of in vitro cell killing by an anti-cancer drug, (5) confirmation that a specific US exposure of cells in vitro inhibits adenylate cyclase activity, (6) discerning the mechanism for US-induced cell lysis in vitro, (7) ascertaining if species-specific differences in erythrocyte cell size and critical shear stress correspond to differences in sensitivity to US-induced hemolysis, and (8) verifying the detectability of US-induced bubbles in the guinea pig hind limb and ascertaining if injected, stabilized microbubbles enhance the frequency of detectable bubbles. Hypotheses (9) and (10) entail developing comprehensive analytic constructs of US-induced kinetics of bubble action under blood vessel confinement to determine if rigorous theory supports the current MI definition, and if cavitation microjets may occur in vivo. The proposed experiments are aimed at elucidating the physical mechanism(s) of action involved in US bioeffect induction, with particular emphasis on the relationships between the US exposure conditions associated with effect induction and the MI as proposed for use in clinical US applications.